1. Field of the Invention
The present invention relates to an ultrasonic actuator used in an autofocus (AF) mechanism incorporated in, for example, an optical device, and to a driving method of the ultrasonic actuator.
2. Description of Related Art
Existing cameras such as a film camera and a digital camera incorporate a lens driver for automatically driving a lens, and various kinds of lens drivers have been hitherto developed. FIG. 1 shows a conventional lens driver using a stepping motor.
As shown in FIG. 1, a conventional lens driver 900 has a zoom mechanism adjusting a focal distance (zoom factor) and an autofocus mechanism automatically adjusting the focus of a lens. These mechanisms are provided with stepping motors 901 and 902, and gears 903 and 904 are mounted to motor shafts of the stepping motors 901 and 902 respectively. The lenses 905 and 906 as a zoom lens and an autofocus lens incorporate gear holders 907 and 908, and gears 903 and 904 are fitted in these gear holders. In the conventional lens driver 900, the motors 901 and 902 of the zoom mechanism and the autofocus mechanism are rotated to drive the lenses 905 and 906. The lenses 905 and 906 are axially supported to a lens holding shaft 909.
Recent compact terminals such as a cell phone or a mobile terminal are equipped with a camera. Hence, there arises a need to downsize the camera, and a size and position of a motor of the lens driver are important factors in size reduction. The conventional lens driver using the stepping motor requires a pair of expensive stepping motors for both of the zoom mechanism and the autofocus mechanism. In addition, there is a limitation on size reduction of the stepping motor itself. From the viewpoint of component arrangement as well, size reduction in actuator is limited. If the components are arranged with intent to reduce a size, the number of components for transmitting a torque from the stepping motor increases, and each mechanism is complicated. Thus, the conventional lens driver using a stepping motor has limitations in size reduction as well as costs high.
To that end, from the purpose of downsizing the lens driver, there has been developed a lens driver using an ultrasonic actuator, not a stepping motor (see Japanese Unexamined Patent Publication No. 2003-33054, Japanese Unexamined Patent Publication No. 2002-303775, and Japanese Unexamined Patent Publication No. 9-298890, for instance). For example, a lens driver as disclosed in Japanese Unexamined Patent Publication No. 2003-33054 drives one lens using a single ultrasonic actuator. In contrast, plural lenses may be also individually driven by plural ultrasonic actuators. In this case as well, a pair of ultrasonic actuators are provided to both of the zoom mechanism and the autofocus mechanism similar to the lens driver using a pair of stepping motors. A lens driver using plural ultrasonic actuators is more expensive than a lens driver using a single ultrasonic actuator, similar to the lens driver using plural stepping motors, and its structure is complicated, so there is a limitation on size reduction.
Here, the ultrasonic actuator is an actuator based on a novel operating principle, not based on an electromagnetic operation as the conventional principle of driving the motor, and is used in a camera lens driver and a medical microactuator. A driving power source of the ultrasonic motor is a piezoelectric vibrator. There are two types of piezoelectric vibrators: a resonant type and a nonresonant type. The resonant type piezoelectric vibrator is used for the ultrasonic actuator of the present invention. There have been known two types of resonant type piezoelectric vibrators: a standing wave type the vibration node of which is not moved, and a traveling wave type the vibration node and loop of which are moved at sound velocity. These two types are based on one-dimensional reciprocating vibrations of a vibrator the gravity center of which is fixed. In addition, there have been utilized a standing wave vibrator that makes a two-dimensional reciprocating motion along the circumference of the cylindrical vibrator the gravity center of which is fixed, based on bending vibrations. Among these, the standing wave makes a reciprocating motion, not moves in one direction. The traveling wave moves in one direction, and can be utilized as a driving force. However, existing traveling wave vibrators are structured such that one of the original positive and negative phases of a standing wave is removed into a traveling wave, so its generation efficiency is low.
On the other hand, the piezoelectric actuator is a revolving resonator that can generate a reciprocal rotational torque at a single-phase alternating voltage, and save vibrating power Q times higher than excitation power (Q is larger than 1,000) and hold the power with as high efficiency as 99.9% or more. The revolving resonator is a torque resonator that means a resonator where electrostriction revolves, in other words, an element called a piezoelectric actuator which generates a torque with the highest efficiency. This element is mistaken for a kind of traveling wave vibrator in some cases. Upon resonance, the vibration loop makes one rotation every cycle along the circumference in sync with an excitation voltage, while the vibration node is not in a vibrating medium and revolves together with the center of gravity of the vibrator. Since the node is not moved, it cannot be of course regarded as a traveling wave, nor a wavemotion. It is, so to speak, a vortex. The vortex is a two-dimensional torque generated through rotation in a combined mode of a circumferential rotation mode and radial rotation mode. If having a cylindrical shape, the resonator is a three-dimensional resonator where an axial rotation mode can be readily combined with these two modes, so three-dimensional mode rotation occurs, and three-dimensional torque of various directions is generated on the entire surface of the piezoelectric actuator, as disclosed in the publication below. The revolving torque is generated with an asymmetric mode for applying voltages of opposite polarities between opposing electrodes A and C and between opposing electrodes B and D. As a result, the gravity center revolves around its center, and the outer circumference of a circle is eccentrically moves like a hula hoop. Then, the piezoelectric actuator moves on spiral trajectories. In this way, the revolving torque is generated.
Japanese Unexamined Patent Publication No. 2002-303775 discloses a structural example of the ultrasonic motor. FIG. 2 is a schematic perspective view of the ultrasonic motor as disclosed in Japanese Unexamined Patent Publication No. 2002-303775. The ultrasonic motor includes a cylindrical piezoelectric ceramic stator 11 and a ring-shaped rotor 12 that is brought into close contact with a rear edge of the stator. Four divided electrodes, electrodes 111, 112, 113, and 114, are formed on the outer peripheral surface of the cylindrical stator 11, and a common electrode (not shown) is formed throughout the inner peripheral surface.
As shown in a schematic diagram of FIG. 3A, alternating driving signals A, B, C, and D that are out of phase are applied to the electrodes 111, 112, 113, and 114. Further, the inner-peripheral-surface electrode of the cylindrical stator 11 is kept at a floating or ground potential (midpoint potential) As shown in FIGS. 3A and 3B, there is a phase shift of 90 degrees between the driving signals A and B, B and C, C and D, and D and A. Accordingly, there is a phase shift of 180 degrees between the driving signal A and the driving signal C and between the driving signal B and the driving signal D. If excited with the voltage asymmetrical with respect to the axis of the cylinder, every portion of the piezoelectric element would expand and contract in accordance with a corresponding voltage level. However, circumferential force acts in adjacent portions, resulting in occurrence of rotation in a combined mode of a radial rotation mode and a circumferential rotation mode. This causes spiral movement. If such spiral movement occurs, resonant phenomenon that the center of gravity moves around its center occurs, and the turning radius is resonance-amplified. In this way, the piezoelectric actuator can directly excite rotation, and strong revolving torque is uniformly generated in the peripheral surface. As for a cylindrical stator, a torque is generated throughout the entire surface, and strong revolving torque perpendicular to the diameter is generated especially at the end surface. The rotational torque can be directly turned into the rotational force of the rotor 12 that is brought into pressure contact with the stator 11.
Referring now to a graph of FIG. 5, a relation between a load current and a drive frequency in an example of the conventional ultrasonic motor is described. If a frequency is decreased from a rotation stop point higher than a resonance frequency of the piezoelectric actuator (piezoelectric element) of the stator 11, the load current is maximized at a predetermined frequency. At this time, the largest torque is obtained. If the frequency is a little lowered from the frequency where the load current is maximized, the ultrasonic motor reaches a resonance step-out point, with the result that the load current abruptly decreases to stop the rotation. Next, in the case of increasing the frequency from the step-out point up to a resonance demodulation point, the load current abruptly increases to resume the rotation. If further increased, the frequency reaches a stop point. It is a feature of non-linear resonance that the step-out point and the demodulation point are not at the same frequency level, the history of change is obtained through frequency sweep, and a current jump phenomenon occurs.
To elaborate, as shown in FIG. 6, in the case of increasing a frequency from a lower level, the ultrasonic motor is kept at rest at a frequency lower than the resonance frequency (recovery frequency). The ultrasonic motor rotates if the frequency exceeds the recovery frequency. The ultrasonic motor stops rotating if the frequency reaches the rotation stop point. In contrast, in the case of decreasing the frequency from a higher level, the ultrasonic motor does not stop rotation even at the frequency beyond the recovery frequency, and stops at the step-out point.
To that end, a resonance frequency of the piezoelectric actuator and a current amount are measured beforehand, and an appropriate driving signal is input to the piezoelectric actuator based on the data. As shown in FIG. 7, operating characteristics of the ultrasonic motor are such that the resonance frequency f0 is shifted to the frequency f1 or f2 due to various factors such as heat generated upon motor driving. Therefore, an amount of current flowing through the piezoelectric actuator at a predetermined frequency is changed, so even if driving signals of constant frequency are input all the time, it is difficult to keep a stationary state. To overcome this problem, reported is a method of feeding back a drive current from the ultrasonic motor, controlling a frequency of the driving signal to an appropriate level, and keeping the maximum current (peak current) (see Japanese Unexamined Patent Publication No. 2002-359988, for instance).
Further, even if a feed-back control on a frequency of the driving signal is executed, the ultrasonic motor has nonlinear characteristics, so there is a fear that the frequency reaches the step-out point to stop the motor rotation. Thus, the ultrasonic motor can be driven while kept in a stable state for a certain amount of time but can be hardly driven while kept in a stable state for a long period. Further, a resonance frequency slightly varies among piezoelectric actuators, so it is difficult to set a single resonance frequency that enables stable operations of many piezoelectric actuators. Furthermore, the piezoelectric actuator has hysteresis characteristic. Therefore, if the same piezoelectric actuators are used, the resonance frequency is changed.
As described above, a frequency history band from the step-out point to the demodulation point is an unstable range as shown in FIG. 5, and is inappropriate as a drive area. A frequency band from the demodulation point to the stop point is appropriate for driving. The frequency history band is unique to a nonlinear resonator in which as resonance amplitude increases to some degrees, saturation phenomenon suddenly occurs, Q drops, and a heat loss occurs. Therefore, the bandwidth varies depending on a frequency sweep speed. On the other hand, the demodulation point is a frequency where the ultrasonic motor returns from a non-oscillation state, that is, a non-heat-generating state to a resonant state, so stable frequency measurement values can be easily obtained. In practice, if the ultrasonic motor is excited to keep a resonant state with a constant frequency around the demodulation point, the resonance frequency is shifted to a higher band due to heat generation, and an exciting frequency is shifted to a history band lower than the demodulation frequency. Finally, the loss of synchronism occurs, and heat generation is stopped. If the ultrasonic actuator is held in this state for a predetermined period, the demodulation frequency corresponds to the exciting frequency due to temperature drop resulting from heat radiation. Then, the ultrasonic actuator jumps to the resonant state. If the exciting frequency is adjusted, a frequency where a jump to the loss of synchronism/demodulation is repeated at regular intervals is found. This frequency is a lower limit of a stable drive area, and a band from the lower limit to the stop point is the stable drive area.
The thus-structured ultrasonic motor has a function of a high-definition stepping motor. In order to obtain various actuators using this motor, it is necessary to lower the motor rpm, that is, decelerate the motor without impairing the high mobility. As conceivable measures for reducing the motor rpm, there is a method of reducing the rpm using a gear or the like in a motor mechanism. However, this method involves a fear that the high mobility is impaired, and the size is increased because of its structure. In particular, this method is insufficient for an application that requires size reduction and high definition, such as an AF actuator.
Further, as conceivable methods of electrically reducing the motor rpm, there is a method of changing the driving signal. However, nonlinearity is high, so a linear response cannot be expected, and controllability is low. For reducing the rpm under constant voltage conditions, there is a method of shifting a driving signal frequency to a lower level in a drive frequency range. This method shifts a frequency to a higher level to reduce a vibration energy generated in the piezoelectric element to reduce an energy transmitted to the rotor to thereby reduce the motor rpm, and thus involves a problem in that a drive torque as well as the rpm reduces along with the reduction in transmitted energy, and it is impossible to change only the rpm under constant output torque conditions.